1. Field of the Invention
The present invention relates to an image capturing device employed in an electronic camera and the like, and more specifically it relates to a light quantity detection required in decision-making with respect to exposure conditions.
2. Description of the Related Art
An image capturing device achieved by two-dimensionally arraying a great number of photoelectric conversion cells (pixels) is employed to convert the image of a subject to an electrical signal in an electronic camera and the like. At each photoelectric conversion cell in the image capturing device, the electrical charge corresponding to the intensity of the incident light and the length of exposure time (the length of time over which the electrical charge is stored) is generated and stored.
This type of image capturing device includes CCD image capturing devices employing CCD elements to constitute photoelectric conversion cells and amplifier-type image capturing devices employing amplifier elements as photoelectric conversion cells. The amplifier-type image capturing devices in the prior art include, for instance, that disclosed in Japanese Laid-Open Patent Publication No. H8-293591. In this prior art technology, junction-type field effect transistors (J-FET) are employed as amplifying elements.
Each photoelectric conversion cell in an image capturing device is provided with a photoelectric conversion portion, a reset portion, an electrical charge transfer portion, an amplifier portion and the like. The reset portion is used for the initialization of the stored electrical charge. The electrical charge transfer portion transfers the electrical charge that has been stored at the photoelectric conversion portion to the amplifier portion.
In this type of image capturing device, the quantity of stored electrical charge at each photoelectric conversion cell is determined in correspondence to the intensity of the incident light and the length of exposure time.
If the illumination of the subject is extremely low, the quantity of the stored electrical charge at each corresponding photoelectric conversion cells is reduced. This results in an increase in the degree of influence of the noise components such as the dark current, which, in turn, leads to poor image quality.
In addition, if the illumination of the subject is extremely high, the adverse effect of the electrical charge flowing out from the high intensity pixel to adjacent pixels results in blooming (bleeding) and smearing in the photographed image.
When the illumination of the subject is low, the degree of the influence of noise can be reduced by increasing the length of exposure time, since this will increase the quantity of stored electrical charge. When the illumination of the subject is extremely high, on the other hand, the length of exposure time may be reduced to keep down the quantity of stored electrical charge so that the electrical charge is prevented from flowing out from a high intensity pixel to adjacent pixels.
In order to achieve such control of the exposure time, it is essential that the exposure state be accurately ascertained. However, in a common charge-storage type solid image capturing device, the charges at the individual photoelectric conversion cells cannot be read out until the actual exposure is completed.
Consequently, in order to detect the photographing environment such as the brightness of the subject, a special exposure meter must be provided. However, since the quantity of scattered reflected light at the detection surface is small in a solid image capturing device, TTL photometry as performed in a camera using silver halide film cannot be implemented.
Thus, it is difficult to accurately measure the intensity of light entering the detection surface of the image capturing device and the exposure quantity. In other words, it is difficult to accurately ascertain the photographing environment unless the state of the image photographed by actually performing photographing is checked.
In addition, in order to perform photographing at a correct exposure for various subjects, it is necessary to detect the quantities of light at a plurality of areas on the photosensitive surface of the image capturing device.